I think this paper will be helpful

https://doi.org/10.1016/j.matchemphys.2020.123236

I would consider reducing the sampling time for the first 15 min of adsorption. I would also shorten the adsorption time. The increase in MB concentration after turning UV on is difficult to explain. It is as if part of the adsorbed MB has been desorbed.

Dear Dinushi,

You need to refer the batch adsorption test procedure,

1. Sorbent dosage optimization (1 g/L, 2 g/L, 5 g/L, 10 g/L), [constant pH, constant conc. of MB, Constant temperature, Constant time]

2. Sorbent pH (Edge): need to obtain good sorbent dosage and all other parameters constant while change the pH.

3. Isotherm (MB dosage), optimum pH, optimum Sorbent dose, MB dose changing while all parameters other parameter constant.

4. Kinetic (contacy time) all other parameter constant and changing contact time.

Once you are getting low absorbance for initial samples, you need more optimization, first think about adsorbent dosage

While you increase adsorbent dosage it will take more time to get equilibrium.

While you decrease MB concentration it will also take more time to get saturation.

While you can reduce the sample collection time (3 min)

likewise you can try for optimization.

Keep in mind to go through control studies, then only we can get an idea about removal process.

I made some example control study in excel try to get an idea from it.

Strongly agree Mirosław Grzesik sir, with suggestions you need to do further optimization.

Thank you.

To try and explain the observed anomaly (increase in MB concentration), more information about the experiment would be helpful. The information provided does not allow this.

Dinushi Munasingha Mudiyanselage, Write, did the experiment proceed as follows? Sample I - 15 min shaking without UV, sample 2 - 30 min shaking without UV, ..., sample 5 - 75 min shaking without UV, sample 6 - 75 min shaking without UV and 15 min with UV, sample 7 - 75 min shaking without UV and 30 min with UV etc. If so, an explanation of the observed concentration (absorbance) changes is possible.